Apparently struct variables have the same naming limitations as blueprints do (no spaces, question marks, etc.) but they don’t have the same warning, it just deletes the name you give if it contains illegal characters with no feedback on what went wrong. If the same warning you get while naming blueprints would show up for naming struct variables, that would be nice!
This is for the binary version of 4.9. (Version: 4.9.1-2689999+++depot+UE4-Releases+4.9 from the ‘about’ menu)